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Relaxation (NMR)

About: Relaxation (NMR) is a research topic. Over the lifetime, 29342 publications have been published within this topic receiving 689851 citations.


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TL;DR: In this paper, simple closed-form approximate expressions for time correlation functions of spherical harmonics with l = 2, m = 0, ±1, and ±2, within the framework of the diffusion in the cone model are presented.
Abstract: Simple closed‐form approximate expressions are presented for time‐correlation functions of spherical harmonics, with l = 2, m = 0, ±1, and ±2, within the framework of the diffusion in the cone model. In this model, a unit vector, with orientation specified by the polar angles ϑ and φ, is restricted to diffuse in a cone of semiangle ϑ0. By comparing with exact numerical results, it is shown that our expressions are highly accurate when m = 0, ±2 for 0°⩽ϑ0⩽90° and when m = ±1 for 0°⩽ϑ0⩽75°. The utility of our results is demonstrated in the context of modeling the effect of hindered motion of probes in membranes on the fluorescence emission anisotropy and the effect of internal motions on the nuclear magnetic resonance relaxation times of protons in nucleic acids.

136 citations

Journal ArticleDOI
TL;DR: In this article, the Johari-Goldstein β process of organic glass formers by one-dimensional and two-dimensional (2D) 2H nuclear magnetic resonance (NMR) was studied.
Abstract: We study the Johari–Goldstein β process of organic glass formers by one- (1D) and two-dimensional (2D) 2H nuclear magnetic resonance (NMR). In particular, we compare systems with pronounced secondary relaxation in dielectric spectroscopy, namely toluene-d5 and polybutadiene-d6 (PB), with compounds which do not exhibit a distinct β peak, i.e., glycerol-d5 and polystyrene-d3 (PS). Choosing large interpulse delays in the applied echo pulse sequences we increase the sensitivity on small angle rotational jumps. This way, we are able to probe clearly the β process of toluene and PB in the line shape of 1D 2H NMR spectra and in the orientational correlation functions of 2D 2H NMR in time domain which is not possible when using the conventional techniques. Below the glass transition temperature Tg, the secondary relaxation of both glass formers is caused by a highly restricted reorientation of essentially all molecules. Comparing our results with simulations we estimate that the reorientation of most toluene mole...

136 citations

Journal ArticleDOI
TL;DR: In this paper, the activation energy was found to correspond to a single torsional barrier and the relaxation times for decay of the torsion angle autocorrelation function were found to display non-Arrhe...
Abstract: Molecular dynamics (MD) simulation has been used to study chain dynamics in bulk polyethylene over a wide range of temperature. This range includes the volumetric glass transition. The latter was determined from the volume temperature relation established via constant pressure MD runs. Conformational transition rates were monitored over the temperature range and found to remain Arrhenius in nature even through the glass transition. The activation energy was found to correspond to a single torsional barrier. Correlation of transitions was monitored as well and, in common with Brownian dynamics simulations of solutions and previous bulk simulations, ±2 neighbor correlations were found to be common. Several new types however were identified. In addition, it was found that ±0 self correlation becomes increasingly important as temperature is lowered. In contrast to the conformational transition rates, the relaxation times for decay of the torsional angle autocorrelation function were found to display non‐Arrhe...

136 citations

Journal ArticleDOI
TL;DR: In this article, rotational correlation times for water molecules were evaluated in mixtures with mol fractions of water, x(H2O), ⩽0.7, interpreted in terms of end-over-end rotation of DMSO molecules and internal rotation of methyl groups.
Abstract: Spin-lattice (T1) and, where appropriate, tranverse (T2) relaxation times are reported for the protons in DMSO + water mixtures over a range of temperature and composition. Self-diffusion coefficients are also measured. A minimum in T1 for a d6-DMSO + H2O sample enables rotational correlation times for water molecules to be evaluated in mixtures with mol fractions of water, x(H2O), ⩽0.7. The low temperature (170-250 K) behaviour of T1 for DMSO + D2O mixtures with x(D2O)≃0.7 is interpreted in terms of end-over-end rotation of DMSO molecules and internal rotation of methyl groups. Under certain assumptions rotational correlation times for the DMSO molecules are obtained from the intramolecular contribution to T1 derived from measurements on ternary mixtures of the type d6-DMSO + DMSO + D2O. These are found to be very similar to those for the water molecules at the same temperature and compositions suggesting that the DMSO and water molecules reorient together. All measurements indicate a minimum in molecular mobility, both rotational and translational, at around a mol fraction of water of 0.65.

136 citations

Journal ArticleDOI
TL;DR: An appreciation of the distribution of relaxation times and the resulting effects upon the KWW constants should be considered to be essential when working with amorphous pharmaceutical materials, especially when attempting to use relaxation time constants for predicting their physical or chemical stability.

136 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
202227
2021652
2020582
2019614
2018638
2017645